Underground storage tank for low temperature liquefied gas

ABSTRACT

An underground storage tank for a low temperature liquefied gas is provided in which a floor slab of the tank comprises an upper floor slab section placed upon excavated bottom of the soil, a lower floor slab section, and a plurality of supporting members interposed between said upper and lower floor slab sections. A side wall of the tank extends to the lower floor slab section in contact therewith and forms an adiabatic space free from the soil inside of the side wall between the upper and lower floor slab sections and the supporting members.

United States Patent [191 Takase et a],

[451 .Nov. 19, 1974 UNDERGROUND STORAGE TANK FOR 2,520,883 8/1950 Komemann et al 220/9 LG ow TEMPERATURE LIQUEFIED GAS 3,068,654 12/1962 Warren 61/.5 3,274,785 9/1966 Lange 220/9 LG Inventors: Kunio Takase, 0hm1ya;Hisashi 3,326,011 6/1967 Sparling 61/.5 Miyakoshi; Mitsuo Ohmura, both of 3,504,496 4/1970 Hnot 61/.5 Nishinomiya, all of Japan 3,748,865 7/1973 Laverman et a] 62/50 [73] Assignees: Osaka Gas Co., Ltd., Osaka-shi,

Osaka; Ohb hLG i, 11, Prtmary Examiner-W. C. Reynolds Osaka, both f, Japan Assistant Examiner-Alex Grosz Filed p 12 1973 Attorney, Agent, or FirmEliot S. Gerber 21 A I. No.: 350 410 l 1 pp [57 ABSTRACT [30] F i A li ti p i i D An underground storage tank for a low temperature Apr 24 1972 Japan 4741092 liquefied gas is provided in which a floor slab of the tank comprises an upper floor slab section placed July 8, l972 Japan, 47 68340 p e a e otto o the so, a ower oor ab [52] U CH 61/ 5 220/9 LG 220]; section, and a plurality of supporting members inter- [511 Cl U12 6 5/00 posed between said upper and lower floor slab sec- [58] Fieid l 5 36 A 5 tions. A side wall of the tank extends to the lower b. 9 A floor slab section in contact therewith and forms an adiabatic space free from thesoil inside of the side [56] References cued wall between the upper and lower floor slab sections UNITED STATES PATENTS and the supporting members.

2,332,227 10/1943 Jackson 52/169 6 Claims, 5 Drawing Figures 1o 5* fl! 1 llbF} l9 .9 l l "*1 o I q -f 9 v1. 1 21 1 II 11 UNDERGROUND STORAGE TANK FOR LOW TEMPERATURE LIQUEFIED GAS This invention relates to an underground storage tank for a low temperature liquefied gas such as methane, ethane, ethylene and propane.

Generally, such storage tanks are constructed in a group in ground, such as reclaimed ground, adjacent to a coast line for convenience of transportation. It is known that the storage tank is preferably constructed in the underground only with its top closure cover exposed outside of the ground to effectively isolate the tank from the outside temperature and to prevent any unforeseen accident caused by leakage of the liquefied gas from the storage tank. However, the ground-water level in the ground adjacent to the coast line is relatively high and the storage tank is so large that the floor slab thereof is located in a level lower than the groundwater level in such ground. Accordingly, in a known storage tank whose floor slab is a single layer structure, when the soil under the floor slab of the storage tank is mainly clay or silt which is easily frozen and, thereby expanded, coldness of the liquefied gas is conducted through the floor slab and freezes the soil with the result that a large uplift is applied against the floor slab. In order to resist against such large uplift, it has been proposed to make the floor slab of the storage tank thicker. However, in such structure, the difference of temperature between the upper surface and the lower surface of the thick floor slab becomes wider, so that a larger thermal stress will be developed in the floor slab. Such thermal stress causes damages, such as cracks, in the storage tank. I

Accordingly, an object of the present invention is to provide a safe and economical underground storage tank for a low temperature liquefied gas in which coldness of the liquefied gas is effectively intercepted so as not to frozen the soil under a floor slab of the tank.

According to the present invention, a floor slab of a storage tank comprises an upper floor slab section, a lower floor slab section upon excavated bottom of the soil below the ground-water level, and a plurality of supporting members interposed between the upper and lower floor slab sections. An annular side wall of the tank extends to the lower floor slab section in contact therewith and, thereby, forms adiabatic space free from the soil inside of the side wall between the upper and lower floor slab sections and the supporting member.

With the presence of the adiabatic space the groundwater is effectively interrupted from entering into the adiabatic space and, the coldness of the liquefied gas is hardly conducted to the soil under the lower floor slab section, so that the storage tank does not receive any uplift thrust caused by freezing of the water and the soil. Preferably, in order to obtain more successful adiabatic effect in the space, a part of the upper and/or lower floor slab sections made of pearlite or adiabatic material may be applied thereto.

More preferably, a heating means is provided for keeping the underside of the lower floor slab section to a temperature higher than a freezing temperature of the soil under the lower floor slab section. The heating means may be an electric heater, a pipe heater through which heating fluid material passes, or a blower for circulating or feeding hot air, which may be provided inside of the lower floor slab section or inside of the adiabatic space. To facilitate the inspection of the heating means for maintenance, a tunnel and a vertical shaft may be provided for leading the adiabatic spaces to the surface of the ground. Thus, the adiabatic effect in the present storage tank is almost permanently insured.

Furthermore, in a preferred structure of the storage tank of the present invention, the annular side wall of the storage tank deeply extends into the ground to a level lower than the lower floor slab section of the tank to prevent the ground-water outside of the annular side wall from coming into the space enclosed by the annular wall and the lower floor slab section, and an artesian pressure reducing system using a perforated drain pipe net is provided for drawing the ground-water from the space. A sand layer may be provided below the lower floor slab section to filter the ground-water. In the storage tank thus constructed, the uplift of the groundwater acting against the lower floor slab section can be reduced to an unobjectionable degree.

The lower floor slab section may be made thick, heavy and strong enough to resist against the uplift of the ground-water. The thickness and weight of the lower floor slab section are determined in proportion to the height of the ground-water level from the lower floor slab section and the diameter of the tank. Though the lower floor slab section is thick in this structure, by means of the adiabatic space between the upper and lower floor slab sections and the heating means, the thick lower floor slab section is not substantially effected by thermal stress caused by temperature difference between the upper and lower surfaces of the lower floor slab section.

Other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments thereof when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a vertical sectional view of an underground storage tank according to a first embodiment of the present invention,

FIG. 2 is a cross sectional view taken along line IIII in FIG. 1,

FIG. 3 is a vertical sectional view of an underground storage tank according to a second embodiment of the present invention,

FIG. 4 is a vertical sectional view of an underground storage tank according to third embodiment of the present invention, and

FIG. 5 is a cross sectional view taken along line VV in FIG. 4.

Referring now to a first embodiment shown in FIGS. 1 and 2, an underground storage tank for a low temperature liquefied gas comprises a top closure cover 22 exposed to outside of the ground, a concrete annular side wall I] deeply extending into the ground, a concrete floor slab 12 provided at a level lower than a groundwater level 10, and insulation 23 for keeping the upper portion of the tank to a low temperature. The annular side wall of the tank comprises an outer side wall lla extending deeply into the ground beyond the floor slab 12 of the tank and an inner side wall 11b contacting to the outer side wall I la and terminating at the floor slab 12. The outer side wall 11a is formed by a slurry wall method, in which a reinforcing bar cage is inserted in an excavated space and a'concrete is placed into the space. This outer side wall 11a effectively interrupts the ground-water outside of the outer wall from coming into the inside thereof, so that, when the next succeeding excavation work for the storage tank is advanced below the underground water level, pumping-out operation is easily performed. Upon the excavated soil is placed a sand layer 18 for filtering the water, which is pumped out. The floor slab 12 of the storage tank comprises a footing 14 formed on the sand layer 18 at the lower end of the inner side wall 11b, a plurality of supporting posts uprightly extending on the footing l4, and an upper floor slab section 13 integrally formed at lower portion of the inner side wall 11b and supported by the posts 15. Formed inside of the inner side wall between the upper floor slab section, supporting posts, and the footing 14 which functions as the lower floor slab section is an adiabatic space 16 free from any soil therein.

A second embodiment shown in FIG. 3 and a third embodiment shown in FIGS. 4 and 5 are substantially same as the first embodiment mentioned above in respects that the floor slab 12 of the storage tank comprises a closed upper floor slab section 13, supporting posts 15, and a lower floor slab section 14, which forms an adiabatic space 16 free from the soil inside of the side wall 11.

In any embodiment, the adiabatic space 16 is formed in the floor slab 12 of the storage tank, so that no excessive thermal stress is developed in the floor slab. Accordingly, even if the floor slab 12 of the storage tank is constructed from a relatively thin upper floor slab section 13 and a relatively thick lower floor slab section 14, the lower floor slab section 14 has a relatively small difference of temperature between the upper and lower surfaces thereof, thereby developing only an unobjectionable thermal stress therein.

In the first and third embodiments, heating elements 17 are provided in the adiabatic space 16. The heating elements 17 in the adiabatic space positively intercept the coldness of the liquefied gas in the tank from being conducted to the soil under the lower floor slab section 14 and keep the temperature of the underside of the lower floor slab section 14 slightly higher than the freezing temperature of the soil, so that the freezing of the soil is surely prevented.

The adiabatic space 16 is communicated with the surface of the ground through a tunnel 20 and a vertical shaft 19, through which operators can reach the space 16 for inspection of the heating conditions and before completion of the storage tank, can carry in and out construction materials.

In the first embodiment, an artesian pressure reducing system using perforated drain pipe nets 26 is provided in the lower floor slab section 14. In FIG. 1, although the artesian pressure reducing perforations 26 are shown as magnified, the perforations actually occupy only about 0.01 percent of the entire area of the lower floor slab section 14. Such artesian pressure redueing perforations are provided when the height of the ground-water level from the lower floor slab section 14 is so high that the lower floor slab section itself cannot resist against the uplift pressure of the groundwater. With the use of the artesian pressure reducing perforations, the ground-water collected inside of the outer side wall 11a is pumped and temporarily stored at the bottom 21 of the vertical shaft 19, from which the water is pumped out on the ground. Thus, the uplift of the ground-water acting against the lower floor slab section can be reduced to an unobjectionable degree.

In the second and third embodiments, the lower end of the outer side wall 11a terminates at the lower end of the lower floor slab section 14 in contact therewith and does not deeply extend in the ground beyond the lower floor slab section 14 as in the first embodiment. The inner side wall 11b of the tank has the lower end terminated at the upper floor slab section 13, so that the lower portion of the outer side wall 11b between the upper and lower floor slab section 13 and 14 defines a part of the adiabatic space 16. In place of the provision of the artesian pressure reducing system 26, the lower floor slab section 14 is made thick, heavy and strong enough to resist against the uplift of the groundwater. The thicker floor slab section 14 is not substantially effected by the thermal stress caused by temperature difference between the upper and lower surfaces thereof. The storage tanks of these embodiments are preferably constructed to a place where the groundwater level is relatively low.

In any embodiment, the storage tank is kept to a predetermined low temperature by insulation 23 suspended from the top closure cover 22 by a number of steel rods.

Though not shown in the drawings, conventional means may be adapted for filling the lower temperature liquefied gas in the tank and for taking out it therefrom. For example, the liquefied gas is fed in the tank along the inner side surface of the tank and is taken out from the bottom portion thereof. Evaporated gas is also extracted from the tank by a known method.

Although the present invention has been described with reference to the preferred embodiments thereof, many modifications and alterations may be made within the spirit of the present invention. For example, an additional insulation may be provided in the side wall of the tank.

What is claimed is:

1. An underground storage tank for a low temperature liquefied gas comprising a top closure cover outside of ground, a concrete annular side wall deeply extending in the ground, a concrete floor slab provided at a level lower than a ground-water level, and an insulation for keeping, at least, the upper portion of said tank to a low temperature, wherein said floor slab comprises an upper floor slab section, a lower floor slab section placed upon the excavated bottom of the soil, and a plurality of supporting members interposed between said upper and lower floor slab sections, said annular side wall extending to said lower floor slab section in contact therewith and forming an adiabatic space free from the soil inside of said side wall between said upper and lower floor slab sections and said supporting members.

2. An underground storage tank as claimed in claim 1, further comprising a heating means provided in said adiabatic space for keeping the underside of said lower floor slab section to a temperature higher than afreezing temperature of soil under said lower floor slab section and a tunnel and a vertical shaft leading said adiabatic space to the surface of the ground.

3. An underground storage tank as claimed in claim 1, wherein said annular side wall of said tank further extends to a level lower than said lower floor slab section and an artesian pressure reducing system is provided through said lower floor slab section for drawing outer side wall deeply extending in the ground beyond said lower floor slab section, and an inner side wall contacting with said outer side wall and terminating at said lower floor slab section in contact therewith.

6. An underground storage tank as claimed in claim 1, wherein said concrete annular side wall comprises an outer side wall whose lower end terminates at said lower floor slab section and in contact therewith, an an inner side wall contacting with said outer side wall and terminating at said upper floor slab section. 

1. An underground storage tank for a low temperature liquefied gas comprising a top closure cover outside of ground, a concrete annular side wall deeply extending in the ground, a concrete floor slab provided at a level lower than a ground-water level, and an insulation for keeping, at least, the upper portion of said tank to a low temperature, wherein said floor slab comprises an upper floor slab section, a lower floor slab section placed upon the excavated bottom of the soil, and a plurality of supporting members interposed between said upper and lower floor slab sections, said annular side wall extending to said lower floor slab section in contact therewith and forming an adiabatic space free from the soil inside of said side wall between said upper and lower floor slab sections and said supporting members.
 2. An underground storage tank as claimed in claim 1, further comprising a heating means provided in said adiabatic space for keeping the underside of said lower floor slab section to a temperature higher than a freezing temperature of soil under said lower floor slab section and a tunnel and a vertical shaft leading said adiabatic space to the surface of the ground.
 3. An underground storage tank as claimed in claim 1, wherein said annular side wall of said tank further extends to a level lower than said lower floor slab section and an artesian pressure reducing system is provided through said lower floor slab section for drawing ground-water from a place enclosed by said lower floor slab section and said side wall.
 4. An underground storage tank as claimed in claim 1, wherein said lower floor slab section is thick enough to resist against the uplift of the groundwater, the thickness and weight of said lower floor slab section being in proportion to the height of the ground-water level from the said lower floor slab section and the diameter of said tank.
 5. An underground storage tank as claimed in claim 1, wherein said concrete annular side wall comprises an outer side wall deeply extending in the ground beyond said lower floor slab section, and an inner side wall contacting with said outer side wall and terminating at said lower floor slab section in contact therewith.
 6. An underground storage tank as claimed in claim 1, wherein said concrete annular side wall comprises an outer side wall whose lower end terminates at said lower floor slab section and in contact therewith, an an inner side wall contacting with said outer side wall and terminating at said upper floor slab section. 